Method and apparatus for candidate list pruning

ABSTRACT

Video signal coding and decoding functions can generate lists of potential candidates to use in coding and decoding, for example, predictors. Video signal coding component candidate undergo operations before potential inclusion in candidate lists. The candidates are checked after being modified by the operations to see if other equal candidates are already in the candidate list. If equal candidates are not in the list, the modified candidates are added to the candidate list. If equal candidates are already in the list, the modified candidates are not added to the list. Operations that can be performed comprise rounding and clipping.

FIELD OF THE INVENTION

The present principles relate to the field of video compression.

BACKGROUND OF THE INVENTION

Video coding organizations strive to develop more efficient codingstandards and tools. Some of these standards are High Efficiency VideoCoding ISO/IEC ITU-T H.265 (HEVC), MPEG-4 Part 10, Advanced Video Coding(MPEG-4 AVC) and Joint Exploration Test Model (JEM) or H.266. Manycoding tools construct lists of possible candidates for such things asmotion vector prediction and intra prediction. The aim is to find thebest candidate in the RDO (Rate Distortion Optimization) sense.

By ensuring the uniqueness of the candidates within these lists, it ispossible to improve the performances of tools with limited size lists,by adding diversity within these lists, and/or reduce the complexity oftools with unlimited lists, for example, by removing identicalcandidates within these lists.

Under the aspects described herein, several embodiments for candidatelist pruning are described.

SUMMARY OF THE INVENTION

These and other drawbacks and disadvantages of the prior art areaddressed by the present aspects, which are directed to a method andapparatus for candidate list pruning to increase the coding efficiencyand/or to reduce the complexity of video codecs by ensuring uniquenessof the different candidates used.

Most of the tools of those video codecs construct lists of candidatesbefore selecting the best one. In order to improve their performancesand/or to reduce their complexity, the candidates of these lists shouldbe unique to ensure a full diversity with the minimal number ofcandidates.

The candidate pruning could be generic, such that it is applied to alltools systematically, or it could also be specific to each set of tools.

According to another aspect of the described embodiments, there isprovided a method for encoding or decoding a video signal usingcandidate list pruning. The method comprises performing at least oneoperation on a coding component candidate of a video signal to generateat least one modified candidate. The method further comprisesdetermining whether the at least one modified candidate is alreadypresent in a candidate list, and adding the at least one modifiedcandidate if the at least one modified candidate is not previouslypresent in the candidate list.

According to another aspect of the described embodiments, there isprovided an apparatus for encoding or decoding a video signal usingcandidate list pruning. The apparatus comprises a memory, and aprocessor configured to encode or decode a video signal using thecandidate list by performing at least one operation on a on a codingcomponent candidate of a video signal to generate at least one modifiedcandidate, by determining whether the at least one modified candidate isalready present in a candidate list, and by adding the at least onemodified candidate if the at least one modified candidate is notpreviously present in the candidate list.

According to another aspect of the described embodiments, there isprovided a non-transitory computer readable medium containing datacontent generated according to any of the encoding method embodiments,or by any of the encoder apparatus embodiments, for playback using aprocessor.

According to another aspect of the described embodiments, there isprovided a signal comprising video data generated according to any ofthe encoding method embodiments, or by any of the encoder apparatusembodiments, for playback using a processor.

According to another aspect of the described embodiments, there isprovided a computer program product comprising instructions which, whenthe program is executed by a computer, cause the computer to carry outthe method of any one of the decoding method or decoding apparatusembodiments.

These and other aspects, features and advantages of the presentprinciples will become apparent from the following detailed descriptionof exemplary embodiments, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an AMVP derivation process of two motion vector predictors.

FIG. 2 shows a merge derivation process of seven predictors and theirassociated reference frame in a reference frame list, and IC flag.

FIG. 3 shows an affine AMVP derivation process of two affine pairs ofmotion vector predictors for a particular reference frame of aparticular reference frame list.

FIG. 4 shows an AMVP derivation process of two unique motion vectorpredictors for a particular reference frame of a particular referenceframe list.

FIG. 5 shows two first steps of an AMVP derivation process of two uniquemotion vector predictors for a particular reference frame of aparticular reference frame list.

FIG. 6 shows two first steps of an AMVP derivation process of two uniquemotion vector predictors for a particular reference frame of aparticular reference frame list.

FIG. 7 shows a merge derivation process of seven unique predictors andtheir associated reference frame in a reference frame list, and IC flag.

FIG. 8 shows an affine AMVP derivation process of two unique affinepairs of motion vector predictors for a particular reference frame of aparticular reference frame list.

FIG. 9 shows a typical encoder to which the described embodiments can beapplied.

FIG. 10 shows a typical decoder to which the described embodiments canbe applied.

FIG. 11 shows one embodiment of a method used for encoding or decoding avideo signal using candidate list pruning.

FIG. 12 shows one embodiment of an apparatus used for encoding ordecoding a video signal using candidate list pruning.

DETAILED DESCRIPTION

An approach for candidate list pruning for video encoding and decodingis described herein.

Most of the tools of video codecs construct lists of candidates beforeselecting the best one to use in the coding or decoding process. Thesecandidates are video coding component candidates used in the coding ordecoding of a video signal. The video components can be elements such asmotion vectors, predictions, quantization parameters, for example. Inorder to improve their performances and/or to reduce their complexity,the candidates of these lists should be unique to ensure a fulldiversity with the minimal number of candidates. The candidate pruningcan be generic in that it can be applied to all tools systematically, orit can also be specific to each set of tools. Some of such tools usinglists are presented hereafter, but the general aspects described hereinare not limited to these tools.

1. AMVP Prediction

The AMVP (Advanced Motion Vector Prediction) prediction consists inconstructing a list of two predictors for each reference frame of eachreference frame list (L0 and L1). These two AMVP predictors are composedof a motion vector.

For a particular reference frame of a particular reference frame list,the predictors come from spatial and/or temporal neighboring blocks(Coding Units/Prediction Units).

Further operations can then be performed on these predictors, such asFRUC (Frame Rate Up Conversion) derivation (i.e. motion vectorrefinement), clipping and/or rounding operations, etc. The process forsuch predictor derivation is as in FIG. 1.

2. Merge prediction

The Merge prediction consists in constructing a set of seven motionvector predictors with their associated reference frame and referenceframe list, and corresponding IC flag (i.e. the local IlluminationCompensation flag).

These seven Merge predictors are composed of:

-   -   A motion vector    -   A reference frame index    -   A reference frame list    -   An IC flag

The predictors come from spatial and/or temporal neighbors from whichthe motion vector and associated reference frame and IC flag are picked.Further operations can then be performed on these predictors, such asclipping and/or rounding operations, for example.

The process for such predictor derivation is as shown in FIG. 2, itstops as soon as seven candidates have been obtained.

3. FRUC Prediction

The FRUC (Frame Rate Up Conversion) process uses the Merge predictors asdescribed in the previous section, but do not use the IC flaginformation of those predictors. The FRUC Merge predictors coming fromthe Merge candidates are composed of:

-   -   A motion vector    -   A reference frame index    -   A reference frame list

The predictors come from spatial and/or temporal neighbors from whichthe motion vector and associated reference frame are selected. Furtheroperations can then be performed on these predictors, such as clippingand/or rounding operations, for example. The process for such predictorderivation is as shown in FIG. 2.

4. Affine AMVP Prediction

The Affine AMVP prediction is quite similar to the one of classicalAMVP, but Affine motion estimation and compensation are performed byusing two motion vectors, called subsequently affine pair of motionvectors. Then, it consists in finding two affine pairs of motion vectorpredictors for each reference frame of each reference frame list (L0 andL1).

The Affine AMVP predictors are composed of:

-   -   A pair of motion vectors        For a particular reference frame of a particular reference frame        list, the predictors come from spatial and/or temporal        neighboring blocks. Further operations can then be performed on        these predictors, such as clipping and/or rounding operations,        etc.        The process for such predictor derivation is as shown in FIG. 3.

5. Intra Prediction

The Intra prediction consists in testing several directional predictions(9 in AVC, up to 35 in HEVC and up to 67 in JEM) in order to find thebest one. In the more recent codecs, several new tools have been addedaround Intra prediction as MPM (Most Probable Mode), RSAF (ReferenceSample Adaptive Filter), PDPC (Position Dependent intra PredictionCombination), for example.

In any case, several or all predictions are tested, thus reducing thisnumber of predictions can decrease the complexity.

The main aspects of the described embodiments is:

-   -   During the construction of any list of candidates, each time a        new candidate is to be added into that list:        -   Perform all operations that can modify this candidate        -   Compare this new candidate to all the already selected ones            in the list        -   Add it to the list if it is not already present

This is done either to diversify the list by ensuring the uniqueness ofeach candidate or to reduce the list size, and so the complexity to dealwith it.

The following embodiments explain how to apply this to each codingmode—1: AMVP, 2: Merge, 3: FRUC, 4: Affine, 5: Intra.

In a first embodiment, the diversity of the AMVP generated list ofpredictors can be improved in several ways.

In a first sub-embodiment of the first embodiment, the AMVP scheme,operations that can modify the candidates should be applied just beforeadding them into the list of predictors.

In the scheme of FIG. 1, these operations stand for the roundingoperations (¼-pel and Integer Motion Vector), but can also be extendedto future ones (clipping, another rounding, etc). These operationsshould be performed together on candidates after they have beencollected and before being added into the list if not already present asshown in FIG. 4.

The “compare/add” function consists in comparing the retrieved candidateto the already selected ones. As an AMVP predictor is composed of only amotion vector, this function compares motion vectors.

In a second sub-embodiment of the first embodiment, the AMVP scheme ofFIG. 1 and FIG. 4, the second spatial candidate does not use theinformation about the first selected one (if some).

It is then possible to inform the second spatial motion vector searchwith the first selected motion vector predictor. The first steps of thescheme then become as shown on FIG. 5.

Since the operation to get a spatial candidate can find one in severalways and stops as soon as one is found, the “different” function standsfor a comparison of the potential candidates to the already selected oneto ensure a different predictor selection.

If the first two sub-embodiments are coupled, it is then also possibleto perform the operations that modify the motion vector predictors(rounding) as soon as a candidate is reached so as to maximize thenumber of different candidates as shown in FIG. 6 and in FIG. 4.

In a second embodiment, the diversity of the Merge generated list ofpredictors can be improved by comparing each new candidate to previouslyselected ones before adding it in the list only if it is not alreadypresent.

FIG. 7 describes this embodiment compared to FIG. 2. On this diagram,the “compare/add” function consists in (i) comparing the just retrievedpredictor to the already selected ones and (ii) adding it if not alreadypresent.

As a Merge predictor is composed of a motion vector, a reference frameindex in a reference frame list and an IC flag, the comparison involvestesting these four components.

In a third embodiment, the diversity of the FRUC candidates coming fromthe Merge ones can be improved (i) by using the same improvements as forMerge, i.e. as presented in FIG. 7, and (ii) by removing the IC flagfrom the Merge process of FIG. 7.

FRUC should use the process from the second embodiment, shown in FIG. 7,to inherit the candidate diversity from the Merge improvements.

To remove the IC flag, the comparisons performed in “compare/add”function and “different” function for spatial candidates should onlyfocus on the three characteristics of the FRUC predictors (motionvector, reference frame index in a reference frame list) instead of thefour ones of the Merge predictors. For that purpose, these functionsmust be informed of the mode used (Merge or FRUC).

In a fourth embodiment, the diversity of the Affine AMVP predictors canalso be improved by ensuring a list of unique predictors.

For that purpose, all the operations that can modify the predictors areperformed before selecting the affine pairs to ensure a full diversity.FIG. 8 shows this process where all triplets of motion vector predictorsare rounded and clipped before selecting the two best ones. The two besttriplets can be the first two different ones with the lower costs, forexample, although other criteria can be used for selecting which onesshould be added as candidates.

If not enough affine predictors have been found, the added AMVPcandidates should also be treated with a “compare/add” function thatmanages comparing the candidate affine pair to a previously selected onebefore adding it, if not already present. As some affine pairscandidates could have been removed because, for example, they are equalto already selected ones, the affine AMVP list can be not full, that is,not filled with two unique affine pairs of motion vectors. It can beuseful to fill this list with a pair of zero motion vectors.

In a fifth embodiment, the number of the Intra predictors can be reducedby removing the redundant ones.

Some reference samples used for directional predictions can be equalbecause either they are the same in the prediction frame or they are notavailable and filled with the first available neighbor.

In that case, several directions may produce the same prediction whichcan easily be inferred. It is then possible to keep only one of thosedirections in the test set. of candidates.

For example, on the left border of a frame, all left reference samplesare filled with the first top sample. Horizontal prediction gives thesame prediction as all bottom-left predictions which can be removed fromthe tested set of candidates. An example is mode 8 for AVC, modes 2 to 9for HEVC, and modes 2 to 18 for JEM (Joint Exploration Test Model).

In a sixth embodiment, several of the previous embodiments can becoupled together. For AVC and HEVC, only the first two sub-embodimentsof the first embodiment, the second embodiment and the fifth embodimentcan be applied. And, for the JEM, all embodiments can be used. In bothcases, any combination of usable embodiments can be observed.

In another embodiment, several embodiments are coupled in the JEM as:

-   -   For AMVP: the first two sub-embodiments of the first embodiment        as described by FIG. 4 and FIG. 5 respectively.    -   For Merge: the second embodiment, described by FIG. 7.    -   For FRUC Merge candidate: the third embodiment, as described by        FIG. 7 associated with an informed “compare/add” function        removing IC flag test.    -   For affine AMVP: the fourth embodiment, as described by FIG. 8.

The described embodiments can be applied both at an encoder and adecoder. At encoding, all candidates are collected and compared in someRDO loops to isolate the best one. At decoding, candidates are collectedin the same way until all possibilities are exhausted or until thesearched one, the best encoded one, is found. For example,

-   -   In AMVP, candidates are collected for each reference frame at        encoding. Then at decoding, the reference frame is known        (transmitted) and candidates are collected for this particular        reference frame in exactly the same way as at encoding.    -   In Merge, seven candidates are collected at encoding, and the        index of the best one is sent. Then at decoding, candidates are        collected in the same way until the index of the searched one is        reached.        So, the processes to collect such candidates are identical both        at an encoding and a decoding side. To speed-up the decoder, a        decoder can stop these processes as soon as the desired        candidate is reached.

FIG. 11 shows one embodiment of a method 1100 for encoding or decoding ablock of video data using candidate list pruning. The method commencesat Start block 1101 and proceeds to block 1110 for performing at leastone operation on a coding component candidate to generate modifiedcoding component candidate(s). Control then proceeds from block 1110 toblock 1120 for determining whether the modified coding componentcandidate(s) are already present in a candidate list. Control thenproceeds from block 1120 to block 1130 for adding the at least onemodified candidate if it is not previously present in the candidatelist.

FIG. 12 shows one embodiment of an apparatus 1200 for coding or decodinga block of video data. The apparatus comprises Processor 1210 which hasinput and output ports and is in signal connectivity with Memory 1220,also having input and output ports. The apparatus can execute any of theaforementioned method embodiments, or variations for encoding ordecoding video signal data using candidate list pruning.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (“DSP”)hardware, read-only memory (“ROM”) for storing software, random accessmemory (“RAM”), and non-volatile storage.

Other hardware, conventional and/or custom, may also be included.Similarly, any switches shown in the figures are conceptual only. Theirfunction may be carried out through the operation of program logic,through dedicated logic, through the interaction of program control anddedicated logic, or even manually, the particular technique beingselectable by the implementer as more specifically understood from thecontext.

The present description illustrates the present principles. It will thusbe appreciated that those skilled in the art will be able to devisevarious arrangements that, although not explicitly described or shownherein, embody the present principles and are included within its spiritand scope.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the presentprinciples and the concepts contributed by the inventor(s) to furtheringthe art, and are to be construed as being without limitation to suchspecifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the present principles, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the block diagrams presented herein represent conceptual views ofillustrative circuitry embodying the present principles. Similarly, itwill be appreciated that any flow charts, flow diagrams, statetransition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedia and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

In the claims hereof, any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementsthat performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Thepresent principles as defined by such claims reside in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. It is thusregarded that any means that can provide those functionalities areequivalent to those shown herein.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

In conclusion, candidate list pruning for video coding and decoding isprovided by at least the described embodiments. The aim is to find thebest candidate in the RDO sense among various candidates during videoencoding and decoding.

By ensuring the uniqueness of the candidates within these lists, it ispossible to improve the performances of tools with limited size lists,by adding diversity within these lists, and/or reduce the complexity oftools with unlimited lists, for example, by removing identicalcandidates within these lists.

1. A method, comprising: encoding a video signal using a candidate listby: performing all motion vector rounding operations on an advancedmotion vector prediction motion vector to generate at least one modifiedcandidate; determining whether said at least one modified candidate ispresent in the candidate list; adding said at least one modifiedcandidate to the candidate list if said at least one modified candidateis not present in the candidate list or not adding said at least onemodified candidate to the candidate list if said at least one modifiedcandidate is present in the candidate list.
 2. A method, comprising:decoding a video signal, comprising: performing all motion vectorrounding operations on an advanced motion vector prediction motionvector to generate at least one modified candidate; determining whethersaid at least one modified candidate is present in a candidate list;adding said at least one modified candidate to the candidate list ifsaid at least one modified candidate is not present in the candidatelist or not adding said at least one modified candidate to the candidatelist if said at least one modified candidate is present in the candidatelist.
 3. An apparatus, comprising: a memory, and a processor configuredto encode a video signal using a candidate list by: performing allmotion vector rounding operations on an advanced motion vectorprediction motion vector to generate at least one modified candidate;determining whether said at least one modified candidate is present inthe candidate list; adding said at least one modified candidate to thecandidate list if said at least one modified candidate is not previouslypresent in the candidate list or not adding said at least one modifiedcandidate to the candidate list if said at least one modified candidateis present in the candidate list.
 4. An apparatus, comprising: a memory,and a processor configured to decode a video signal using a candidatelist by: performing all motion vector rounding operations on an advancedmotion vector prediction motion vector to generate at least one modifiedcandidate; determining whether said at least one modified candidate ispresent in the candidate list; adding said at least one modifiedcandidate to the candidate list if said at least one modified candidateis not previously present in the candidate list or not adding said atleast one modified candidate to the candidate list if said at least onemodified candidate is present in the candidate list.
 5. The method ofclaim 1, wherein said at least one operation comprises clipping,rounding, and selecting a different candidate.
 6. The method of claim 1,wherein said video signal coding component candidate comprises at leastone of motion vector, reference blocks, reference frames, referenceframe index, and a reference frame list.
 7. The method of claim 1,wherein said at least one operation comprises rounding motion vectors toone-quarter pixel resolution and integer motion vector rounding of bothpredictors in an Advanced Motion Vector Prediction process.
 8. Themethod of claim 7, wherein a second motion vector is obtained in an areanot considered as a candidate for a first motion vector.
 9. The methodof claim 1, wherein a merge predictor comprising a motion vector, areference frame index, a reference frame list and an IC flag, are thecandidates to be added to said candidate list if not previously presentin said candidate list.
 10. The method of claim 1, wherein candidates ina frame rate up conversion process are found, comprising a motionvector, a reference frame index, and a reference frame list.
 11. Anon-transitory computer readable medium containing data contentgenerated according to the method of claim 1, for playback.
 12. Acomputer program product comprising instructions which, when the programis executed by a computer, cause the computer to carry out the method ofclaim
 2. 13. The apparatus of claim 3, wherein said at least oneoperation comprises clipping, rounding, and selecting a differentcandidate.
 14. The apparatus of claim 4, wherein said at least oneoperation comprises clipping, rounding, and selecting a differentcandidate.